Repeated cocaine administration sometimes results in tolerance and sometimes in sensitization to the drug's subsequent effects. Variables responsible for this dichotomous set of outcomes have yet to be elucidated fully, but it is clear that behavioral factors may play a role. In preliminary work we found that the amount of responding required for reinforcement could modulate the degree of tolerance that developed when cocaine was administered repeatedly. In the research proposed here we shall analyze this effect and examine its generality. Specifically, the roles of reinforcement frequency and "effort" will be analyzed. In addition, interspecies generality (pigeons vs. squirrel monkeys) will be assessed as will the generality of the effect to other contingency arrangements. Identification and characterization of behavioral factors that modulate the development of tolerance to cocaine's effects could have important implications for treatment and prevention of abuse.